Dark Galaxies Observed For First Time

An international team of astronomers believe they have detected these elusive objects by observing them as they were lit up by glowing quasars.

Dark galaxies are very small, gas-rich galaxies in the early universe that are inefficient at forming stars. Scientists believe these galaxies are building blocks of today's bright, star-filled galaxies.

Astronomers think that these galaxies may have fed large galaxies with much of the gas that later formed into the stars that exist today.

Dark galaxies do not emit much light because they are nearly devoid of stars, making them hard to detect.

Astronomers have been trying to develop new techniques to confirm the existence of these galaxies, and this new study marks the first time the objects have been seen directly.

The team used ESO's VLT to detect the extremely faint fluorescent glow of the dark galaxies. They used the FORS2 instrument to map a region of the sky around the bright quasar, looking for the ultraviolet light that is emitted by hydrogen gas when it is subjected to intense radiation.

“After several years of attempts to detect fluorescent emission from dark galaxies, our results demonstrate the potential of our method to discover and study these fascinating and previously invisible objects,” Sebastiano Cantalupo, lead author of the study, said in a press release.

The team detected nearly 100 gaseous objects which lie within a few million light-years of the quasar. They narrowed down their search to 12 objects after a careful analysis designed to exclude objects where emission might be powered by internal star-formation in the galaxies rather than light from the quasar.

The astronomers were able to determine some of the properties of the dark galaxies. They believe the mass of the gas in them is about 1 billion times that of the Sun.

They were able to estimate that the star formation efficiency is suppressed by a factor of more than 100 relative to typical star-forming galaxies found at a similar stage in cosmic history.

“Our observations with the VLT have provided evidence for the existence of compact and isolated dark clouds. With this study, we´ve made a crucial step towards revealing and understanding the obscure early stages of galaxy formation and how galaxies acquired their gas”, Sebastiano Cantalupo said in the release.